Ultra wideband technology is distinguished from narrowband and spread spectrum technologies in the sense that the bandwidth of an ultra wideband type signal is typically between about 25% to 100% of the central frequency. Moreover, instead of transmitting a continuous carrier modulated with information or with information combined with a spreading code, which determines the bandwidth of the signal, ultra wideband technology involves transmission of a series of very narrow pulses. For example, these pulses may take the form of a single cycle having a pulse width of less than 1 ns. These pulses are extremely short in the time domain, and when transformed into the frequency domain, produce the ultra wideband spectrum that is characteristic of UWB technology.
In UWB technology, the information on the signal can be coded, for example, by a modulation technique called pulse position modulation (PPM). In other words, the information coding is carried out by varying the instant of transmission of individual pulses. More specifically, the pulse train is transmitted at a frequency that can be as much as several tens of MHz. Each pulse is transmitted in a window of predetermined length, such as 50 ns, for example. Compared to a theoretical position of transmission, the pulse is then advanced or delayed, enabling a 0 or a 1 to be coded. More than two values can also be coded by using more than two positions offset relative to the reference position. It is even possible to superimpose a BPSK modulation on this modulation.
On receipt of the transmitted signal, it is therefore necessary to decode these pulses so as to determine the value of the digital information conveyed. This decoding is essentially performed in an analog manner by using an analog correlator. This requires a relatively complex hardware implementation. Moreover, the correlation system is assigned to a fixed position within each pulse transmission window. Consequently, the number of correlation chains must be equal to the number of position modulation levels.
Furthermore, such an architecture requires a precise synchronization to find the right position modulation and the correct polarity of the various symbols. This synchronization is performed by very complex software, since only a limited number of observations are possible. In addition, the precision of the clock, which is typically a few picoseconds, is a very constraining parameter. This is based upon the technology and the current consumed using the technology.